1. Field of the Invention
The present invention relates generally to a receiving apparatus and method in a Broadband Wireless Access (BWA) system, and in particular, to an apparatus and method for enhancing the demodulation performance of a receiver in a multi-cell BWA system.
2. Description of the Related Art
As generally known in the art, communication systems have been primarily developed for voice communication services, but the communications systems are also evolving to provide data services and various multimedia services. However, conventional communication systems, which are mainly directed to providing voice communication services, still have a narrow data transmission bandwidth and require a high subscription fee. For these reasons, the conventional communication systems cannot satisfy diversified user demands. Furthermore, in line with rapid development in the communication industry and ever-increasing demands on Internet services, it is important to provide communication systems capable of providing Internet services efficiently. As a result of these trends, BWA systems having a bandwidth large enough to both satisfy the increasing user demands and provide efficient Internet services have been proposed.
In addition to providing voice communication services, BWA systems also support a combination of various data services both at low and high speeds and multimedia application services (e.g., high-quality moving pictures). BWA systems are based on wireless media using a broadband of 2 GHz, 5 GHz, 26 GHz, or 60 GHz and are able to access a Public Switched Telephone Network (PSTN), a Public Switched Data Network (PSDN), an Internet network, an International Mobile Telecommunications-2000 (IMT-2000) network, and an Asynchronous Transfer Mode (ATM) network in a mobile or stationary environment, i.e., BWA systems are wireless communication systems capable of supporting a channel transmission rate of at least 2 Mbps. BWA systems may be classified into broadband wireless local loops, broadband mobile access networks, and high-speed wireless Local Area Networks (LANs) according to the terminal's mobility (stationary or mobile), communication environment (indoor or outdoor), and channel transmission rate.
The standardization of wireless access schemes of BWA systems is being conducted by the Institute of Electrical and Electronics Engineers (IEEE), particularly by the IEEE 802.16 standardization group, which is an international standardization organization.
IEEE 802.16 communication systems have a larger data transmission bandwidth than conventional communication systems for voice communication services. Therefore, the IEEE 802.16 communications systems can transmit more data for a limited period of time and share all user channels (or resources) for efficient channel utilization. Since Quality of Service (QoS) features are guaranteed, users are provided with various services of different qualities based on the characteristics of services.
The IEEE 802.16 communication systems use an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme for physical channels, i.e., the BWA systems use an OFDM/OFDMA scheme to transmit physical channel signals over subcarriers, thereby enabling high-speed data transmission.
The BWA systems support the mobility of a Mobile Station (MS) through a multi-cell structure and using the same frequency in all the cells for frequency utilization efficiency (spectral efficiency). The performance of such a multi-cell communication system is affected greatly by an inter-cell interference.
FIG. 1 is a schematic diagram of a multi-cell BWA system.
Referring to FIG. 1, a user terminal 110 in a cell-overlap region transmits an interference signal to a neighboring cell 120 in a multi-cell environment with a frequency reuse factor of 1. This interference signal affects a signal of another user terminal in the neighboring cell 120, degrading the demodulation performance. Thus, the inter-cell interference technique must be used in the multi-cell system.
FIG. 2 is a diagram illustrating an inter-cell interference in the multi-cell BWA system, which is shown on the frequency axis. As illustrated in FIG. 2, the inter-cell interference is modeled as a discontinuous narrowband signal in the frequency band.
It is known in the art that a better decoding performance can be provided when a hard-decision value of coded bits, instead of a soft-decision value, is provided to a channel decoder. The input soft-decision value of the decoder is an estimated value of a modulation symbol transmitted on a channel, which may be a Log Likelihood Ratio (LLR). A noise value of a corresponding frequency band is needed to calculate the LLR.
In the conventional art, the LLR is calculated using the average noise value of the entire band, which fails to consider an inter-cell interference with narrowband/discontinuous characteristics illustrated in FIG. 2. This degrades the demodulation performance of a receiver. Typically, noise estimation is performed by using subtraction between neighboring pilot signals (or symbols). Also, an average is taken over the entire band, thereby increasing the estimation accuracy. However, the estimated noise value for the entire band is unsuitable for an OFDM-based BWA system with narrowband interference. What is therefore required is a noise estimator capable of reflecting the narrowband/discontinuous noise characteristics accurately.
As described above, elimination of the multi-cell interference and accurate noise estimation are required in order to enhance the demodulation performance of a receiver in the BWA system.